A problem associated with solar hot water systems is, if the ambient temperature falls below about 6° C., the liquid in the solar panels can expand and thus damage the panels.
To prevent this occurring, one known arrangement involves the use of a heated water storage tank that has an additional storage tank in its interior, the interior storage tank containing water, glycol or another liquid anti-freezing agent. The interior storage tank is in a heat exchanging relationship with the heated water storage tank. A pump is used to drive the fluid into one or more solar panels, where it undergoes heating, and then back to the interior storage tank for heating of the water in the water storage tank. If the ambient temperature falls to about 6° C., the fluid in the solar panels is drained into the interior storage tank. This is known as the “drain-back” method and has several disadvantages. Firstly, it is a complicated and expensive arrangement to manufacture and install, particularly the requirement for the separate interior tank within the water storage tank. Further, when using a liquid other than potable water or a (relatively expensive) food-grade anti-freezing agent, then the interior tank must be of double walled construction. This adds to complexity and cost. Secondly, heat transfer from the fluid to the water is via the tank walls, which is relatively inefficient. Thirdly, the interior tank must be larger than is required for normal operation, in order to be able to store the volume of the emptied solar panel(s), which increases the size of the water storage tank, and thus the system overall. Fourthly, the pump required is relatively expensive as it must be able to lift the fluid against the pressure head in order to refill the solar panels.
Another known arrangement involves circulating glycol, or another liquid anti-freezing agent, from one or more solar panels to one side of a heat exchanger. The other side of the heat exchanger is connected to water circulated from a hot water storage tank. The glycol prevents the panels from being damaged when the temperature falls to below about 6° C. Whilst this arrangement avoids the complicated dual tank arrangement required for the drain-back method, it nonetheless has disadvantages. Firstly, it is also relatively complicated and expensive to manufacture and install. For example, the entire system must be bled of any air, pressurised and then sealed prior to operation. Secondly, the heat transfer between the glycol and the water circuits, through the heat exchanger, is relatively inefficient.
A third known arrangement, known as a recirculating system, uses a solar panel and a heated water storage tank connected in series. In this arrangement, when the temperature falls to about 6° C., a pump is activated to recirculate heated water from the heated water storage tank through the panels to keep them above a temperature where damage will occur. A disadvantage of this arrangement is that some of the energy imparted to the water from the Sun is lost when that heated water is used to warm the panels, which reduces energy efficiency. This arrangement can also utilise a temperature sensitive frost valve that opens when the ambient temperature gets to about 6° C. to drain water from the solar panel back. The draining water also warms the valve which then closes. The disadvantages of this arrangement are, firstly, the additional cost of the valve and increased installation complexity. Secondly, the valve is also prone to blocking and other failure, and has a slow reaction time which may not prevent damage during snap freezing. Thirdly, successful operation is dependant on unimpeded flow in the solar panels and associated pipework.